1. Field of the Invention
The present invention relates to an elastic surface acoustic wave filter device.
2. Description of the Related Art
Recently, with mobile body terminals becoming smaller and less expensive, it has become desirable to reduce the number of parts in mobile body terminals and to combine several parts into an integral composite unit in the circuit configuration. Recent elastic surface acoustic wave filter devices have succeeded in reducing insertion loss and have begun to be provided in antenna top units as well.
FIG. 17 shows one example of an elastic surface acoustic wave filter device used in mobile body terminals, taking the reception side (RX) as an example. Generally, an RF signal is input from an antenna (ANT) through a switch (SW) to an elastic surface acoustic wave filter device (SAW) with unbalanced terminals and a characteristic impedance of 50 .OMEGA.. On the other hand, an amplifier (LNA) is an operational amplifier and has balanced terminals in many cases. This means that the amplifier requires a characteristic impedance of about 150-200 .OMEGA. to receive an input if unbalanced terminals are used. However, most elastic surface acoustic wave filter devices (SAW) usually have an input/output impedance of 50 .OMEGA. within the SAW device and unbalanced input/output terminals. For this reason, although elastic surface acoustic wave filter devices can be directly connected to the antenna (ANT) side, a matching circuit for impedance conversion and an unbalanced-to-balanced transformer have been required for connection to the amplifier (LNA) side. A balun circuit (hereinafter referred to simply as a balun) has been used for that function, but the use of a balun has increased the number of parts and cost. There has been thus a demand for eliminating the use of a matching circuit and an unbalanced-to-balanced transformer.
Recently, a variety of studies have been made to satisfy the above demand. Japanese Unexamined Patent Publication No. 7-288442, for example, achieves a filter of balanced input-to-balanced output type by connecting elastic surface acoustic wave resonators in a symmetrical grid pattern.
FIG. 18 shows the construction of an elastic surface acoustic wave filter device disclosed in the above-cited Japanese Unexamined Patent Publication No. 7-288442. As shown in FIG. 18, an elastic surface acoustic wave filter device 100 comprises two serial arm resonators 101, 101 and two parallel arm resonators 102, 102. Input terminals of one serial arm resonator 101 and one parallel arm resonator 102 are connected to an input terminal 105, and input terminals of the other serial arm resonator 101 and the other parallel arm resonator 102 are connected to an input terminal 106.
Output terminals of one serial arm resonator 101 and the other parallel arm resonator 102 are connected to an output terminal 107, and output terminals of the other serial arm resonator 101 and one parallel arm resonator 102 are connected to an output terminal 108. Also, an inductor 109 is connected in series to each of the four resonators 101, 102.
The elastic surface acoustic wave filter device of balanced input-to-balanced output type is realized with the above construction. But the disclosed elastic surface acoustic wave filter device cannot be used, as it is, to a system having a unbalanced input-to-balanced output. As described in "A New Balanced to Unbalanced Type RF-Band SAW Filter", 1996 IEEE MTT-S Digest WE1A-5 (p 417-420) and Japanese Unexamined Patent Publication No. 8-65098, therefore, a filter of an unbalanced input-to-balanced output type is realized by connecting one of the input IDTs to a ground terminal and the output IDTs to the input terminals 105, 106 of the elastic surface acoustic wave filter device shown in FIG. 18.
Although a filter of unbalanced input-to-balanced output type can be realized according to the method of the above-cited Japanese Unexamined Patent Publication No. 8-65098, the elastic surface acoustic wave filter device still has a conventional characteristic impedance therein. Therefore, equal input and output impedance values such as 50 .OMEGA.--50 .OMEGA., and a matching device for impedance matching is required for connection to the amplifier side having input impedance of about 150-200 .OMEGA.. Further, because the balanced output can be only realized in a floating type device, there arises a problem that a degree of balance outside the band is not complete and direct incoming waves cannot be sufficiently canceled out or suppressed.
An elastic surface acoustic wave filter device of a floating type experiences a problem in that the presence of a ground terminal in the filter input side affects an output terminal and a filter characteristic is varied from a desired value.
Additionally, to achieve connection to the amplifier side without a matching circuit, the impedance on the output side should not be equal to the impedance on the input side, but is required to be about 150-200 .OMEGA., i.e., about three or four times as much as the input impedance. A problem of increasing mismatching loss would result if attempting to achieve such an unbalanced impedance construction is done only by improving a design of an elastic surface acoustic wave filter itself.